1. Field of the Invention
This invention relates to a highly durable and heat resistant water-absorbing composite which comprises an inorganic powder having particle surfaces covered totally by a highly absorbent resin which is obtained from a polymer containing as a monomeric constituent an .alpha.,.beta.-unsaturated compound having in its molecule one or two carboxyl groups, or one or two other groups convertible to a carboxyl group or groups, by reacting the polymer with a basic substance and crosslinking the reaction product with a polyamine. The composite is useful as a water retaining, absorbing or stopping agent, a dehydrating agent, a drying agent, or a wetting agent.
2. DESCRIPTION OF THE PRIOR ART
Various types of highly absorbent resins which can absorb several hundred times as much water as their own weight have recently been developed, and come to be used as a water retaining, absorbing or stopping agent, a drying agent or a wetting agent, or for other purposes. When these resins have absorbed water, however, they are sometimes to low in strength to exhibit their capacity satisfactorily. Moreover, they are almost equal to water in specific gravity when they have absorbed water. If any such resin is, for example, used for dehydrating oil, therefore, it is difficult to separate the water-bearing resin from the oil. In order to overcome this difficulty, Japanese laid-open patent specification No. 5022/1981, for example, proposes the use of a mixture of a highly absorbent resin powder and an organic or inorganic carrier which adhere to each other. The resin and the carrier, however, adhere to each other only at points of contact, and are very liable to separation from each other when their mixture is packed or transported, or mixed with any other material. Even if they remain adherent to each other on any such occasion, their points of contact are severed by the pressure created when they have absorbed water and become swollen. If the mixed material is, for example, used with soil in agriculture or horticulture, the absorption of water by the resin causes it to be separated from the carrier, and the pressure created by the swollen resin develop open spaces in the soil. The resin moves into such spaces, and is precipitated on the surface of the soil. The resin, thus, covers the surface of the soil around the roots of plants, and hinders the supply of sufficient air to the roots, resulting in their rotting.
If any such mixed material is used for dehydrating oil, the absorption of water by the resin causes it separation from the carrier, leaving a mere mixture of the resin and the carrier. The resin floats in the oil, and is difficult to separate therefrom, resulting in inefficient dehydration of oil.
In order to overcome these disadvantages, Japanese laid-open patent specification No. 91086/1978 proposes the covering or mixing of a water-insoluble base, such as paper, cotton, wood, cloth, a synthetic resin, soil, metal or glass, by or with a highly absorbent hydrogel composed mainly of the saponified product of an ethylene-vinyl ester-ethylene type unsaturated carboxylic acid copolymer to render the base capable of absorbing or retaining water. According to this patent specification, a hydrogel in the form of, for example, a film, fibers, beads or a fine powder is mixed with the base, or a water-soluble polymer capable of forming such a hydrogel is dissolved in water or the like, and mixed in the presence of a specific crosslinking agent, and the solution thereby obtained is coated on the base and dried. These materials, however, form a paste with the solubilization of the resin if they retain water for a long time, probably because a dialdehyde, epihalohydrin, diepoxy, triepoxy, diisocyanate or like compound is used as a crosslinking agent. Moreover, they lack durability under heat. It is, therefore, impossible to use them as a water retaining agent for agriculture or horticulture which is required to maintain its water absorbing power for six or twelve months or a longer time, while permitting transmission of air.